The RET proto-oncogene, encoding a receptor tyrosine kinase, is the susceptibility gene for Hirschsprung disease (HSCR), a congenital absence of enteric ganglia, and for multiple endocrine neoplasia type 2 (MEN 2), an inherited cancer syndrome and neurocristopathy. MEN 2 are characterized by medullary thyroid carcinoma (MTC), pheochromocytoma and parathroid hyperplaisa. Four related ligands are needed to bind to four related coreceptors before binding to RET. High penetrance gain of function mutations cause MEN 2 and loss of function mutations cause a subset of HSCR (30-50 percent of familial and 3 percent of isolated cases). Recently, the PIs have found 2 polymorphisms, A45A and L769L within the RET gene that are associated with HSCR in small sample. In contrast S836S are over-represented in individuals with sporadic MTC, particularly those with M918T polymorphism in their tumors. Based on these preliminary genetic findings, the PIs hypothesize that common low penetrance alleles within RET can act as low level susceptibility alleles predisposing to HSCR. Further, they hypothesize that the variants associated with HSCR will be underrepresented in sporadic MTC cases and vice versa. To test these hypotheses, the PIs will accrue a large population-based series of HSCR cases (n=250) and sporadic MTC (n=200) to determine the frequency of polymorphic variants in RET and to determine their haplotypes. Race-matched, region-matched normal controls will be accrued with cases: control ratio of 1:3. These data will be analyzed using the standard case-control matched association analysis, the transmission-disequilibrium test, and the haplotype-based linkage disequilibrium analysis. These methods have been used on a pilot series of 64 HSCR and unaffected parents and found evidence for a novel low penetrance locus with 15kb upstream and including codon 45 which could predispose to the majority of isolated HSCR. The PIs plan to extend their investigations to isolate this new locus and to directly test this locus in the extended series of HSCR. Further, the genes encoding the ligands and coreceptors of RET are equally good candidates for low penetrance susceptibility genes. Genotyping and haplotyping of these genes and similar tests of association will be performed in HSCR and MTC. Finally variants found to be associated with disease status will be tested functionally at the transcript, protein and cell biological level.